craig's restorative dental materials 13th ed. - r. sakaguchi, j. powers (mosby, 2012)

Mitchell, PhD Associate ProfessorDivision of Biomaterials and BiomechanicsDepartment of Restorative DentistrySchool of Dentistry Oregon Health and Science UniversityPortland, Oregon Chap

RESTORATIVE DENTAL

Philadelphia, PA 19103-2899

CRAIG’S RESTORATIVE DENTAL MATERIALS ISBN: 978-0-3230-8108-5

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Library of Congress Cataloging-in-Publication Data

Craig’s restorative dental materials / edited by Ronald L Sakaguchi, John M Powers 13th ed.

p ; cm.

Restorative dental materials

Order of editors reversed on prev ed.

Includes bibliographical references and index.

ISBN 978-0-323-08108-5 (pbk : alk paper) 1 Dental materials I Sakaguchi, Ronald L

II Powers, John M., 1946- III Title: Restorative dental materials

[DNLM: 1 Dental Materials 2 Dental Atraumatic Restorative Treatment WU 190]

RK652.5.P47 2012

617.6’95 dc23

2011015522

Vice President and Publishing Director: Linda Duncan

Executive Editor: John J Dolan

Developmental Editor: Brian S Loehr

Publishing Services Manager: Catherine Jackson/Hemamalini Rajendrababu

Project Manager: Sara Alsup/Divya Krish

Designer: Amy Buxton

Printed in United States

Last digit is the print number: 9 8 7 6 5 4 3 2 1

with whom we have collaborated.

University of São Paulo

São Paulo, SP, Brazil

Chapter 5: Testing of Dental Materials and Biomechanics

Chapter 13: Materials for Adhesion and Luting

Isabelle L Denry, DDS, PhD

Professor

Department of Prosthodontics and Dows Institute

for Dental Research

College of Dentistry

The University of Iowa

Iowa City, Iowa

Chapter 11: Restorative Materials—Ceramics

Jack L Ferracane, PhD

Professor and Chair

Department of Restorative Dentistry

Division Director, Biomaterials and Biomechanics

Professor and Chair

Department of Restorative Dentistry

College of Dentistry

University of Oklahoma Health Sciences Center

Oklahoma City, Oklahoma

Chapter 2: The Oral Environment

David B Mahler, PhD

Professor Emeritus

Division of Biomaterials and Biomechanics

Department of Restorative Dentistry

School of DentistryUniversity of California San FranciscoSan Francisco, California

Chapter 2: The Oral Environment

Sally J Marshall, PhD

Vice Provost, Academic AffairsDirector of the Office of Faculty Development and Advancement

Distinguished Professor Division of Biomaterials and Bioengineering

Department of Preventive and Restorative Dental Sciences

School of DentistryUniversity of California San FranciscoSan Francisco, California

Chapter 2: The Oral Environment

John C Mitchell, PhD

Associate ProfessorDivision of Biomaterials and BiomechanicsDepartment of Restorative DentistrySchool of Dentistry

Oregon Health and Science UniversityPortland, Oregon

Chapter 6: Biocompatibility and Tissue Reaction to Biomaterials

Chapter 15: Dental and Orofacial Implants Chapter 16: Tissue Engineering

Sumita B Mitra, PhD

PartnerMitra Chemical Consulting, LLCWest St Paul, Minnesota

Chapter 9: Restorative Materials—Polymers Chapter 13: Materials for Adhesion and Luting

Kiersten L Muenchinger, AB, MS

Program Director and Associate ProfessorProduct Design

School of Architecture and Allied ArtsUniversity of Oregon

Eugene, Oregon

Chapter 3: Design Criteria for Restorative Dental Materials

Carmem S Pfeifer, DDS, PhD

Research Assistant Professor

Department of Craniofacial Biology

School of Dental Medicine

University of Colorado

Aurora, Colorado

Chapter 4: Fundamentals of Materials Science

Chapter 5: Testing of Dental Materials and Biomechanics

John M Powers, PhD

Editor

The Dental Advisor

Dental Consultants, Inc

Ann Arbor, Michigan

Professor of Oral Biomaterials

Department of Restorative Dentistry

and Biomaterials

UTHealth School of Dentistry

The University of Texas Health Science Center

at Houston

Houston, Texas

Chapter 12: Replicating Materials—Impression and Casting

Chapter 14: Digital Imaging and Processing for

Restorations

Ronald L Sakaguchi, DDS, MS, PhD, MBA

Associate Dean for Research and InnovationProfessor

Division of Biomaterials and BiomechanicsDepartment of Restorative DentistrySchool of Dentistry

Oregon Health and Science UniversityPortland, Oregon

Chapter 1: Role and Significance of Restorative Dental Materials

Chapter 3: Design Criteria for Restorative Dental Materials Chapter 4: Fundamentals of Materials Science

Chapter 5: Testing of Dental Materials and Biomechanics Chapter 7: General Classes of Biomaterials

Chapter 8: Preventive and Intermediary Materials Chapter 9: Restorative Materials—Composites and Polymers Chapter 10: Restorative Materials—Metals

Chapter 14: Digital Imaging and Processing for Restorations

Chapter 15: Dental and Orofacial Implants

Preface

The thirteenth edition of this classic textbook has

been extensively rewritten to include the many

recent developments in dental biomaterials science

and new materials for clinical use One of our goals

for this edition is to include more clinical

applica-tions and examples, with the hope that the book will

be more useful to practicing clinicians The book

con-tinues to be designed for predoctoral dental students

and also provides an excellent update of dental

bio-materials science and clinical applications of

restor-ative materials for students in graduate programs

and residencies

Dr Ronald L Sakaguchi is the new lead editor of

the thirteenth edition Dr Sakaguchi earned a BS in

cybernetics from University of California Los Angeles

(UCLA), a DDS from Northwestern University, an MS

in prosthodontics from the University of Minnesota,

and a PhD in biomaterials and biomechanics from

Thames Polytechnic (London, England; now the

Uni-versity of Greenwich) He is currently Associate Dean

for Research & Innovation and a professor in the

Divi-sion of Biomaterials & Biomechanics in the

Depart-ment of Restorative Dentistry at Oregon Health &

Science University (OHSU) in Portland, Oregon

Dr John M Powers is the new co-editor of the

thirteenth edition He served as the lead editor of the

twelfth edition and contributed to the previous eight

editions Dr Powers earned a BS in chemistry and a

PhD in mechanical engineering and dental materials

at the University of Michigan, was a faculty member

at the School of Dentistry at the University of

Michi-gan for a number of years, and is currently a professor

of oral biomaterials in the Department of Restorative

Dentistry and Biomaterials at the UTHealth School

of Dentistry, The University of Texas Health Science

Center at Houston He was formerly Director of the

Houston Biomaterials Research Center Dr Powers is

also senior vice president of Dental Consultants, Inc.,

and is co-editor of The Dental Advisor.

The team of editors and authors for the thirteenth

edition spans three generations of dental

research-ers and educators Dr Sakaguchi received his first

exposure to dental biomaterials science as a first-year

dental student at Northwestern University Dental

School Drs Bill and Sally Marshall were the

instruc-tors for those courses After many years of

men-toring received from Drs Bill Douglas and Ralph

DeLong, and Ms Maria Pintado at the University

of Minnesota, Dr Sakaguchi joined the biomaterials

research team in the School of Dentistry at OHSU with

Drs David Mahler, Jack Mitchem and Jack Ferracane The OHSU laboratory benefited from the contributions

of many visiting professors, post- doctoral fellows, and graduate students, including Dr Carmem Pfeifer who conducted her PhD research in our laboratory Thanks

to the many mentors who generously contributed directly and indirectly to this edition of the book

We welcome the following new contributors to the thirteenth edition and thank them for their effort and expertise: Drs Bill and Sally Marshall of University of California San Francisco (UCSF); Dr Sumita Mitra of Mitra Chemical Consulting, LLC, and many years at 3M ESPE; Dr Jack Ferracane of OHSU; Dr Roberto Braga of the University of São Paulo; Dr Sharukh Khajotia of the University of Oklahoma; Dr Carmem Pfeifer of the University of Colorado, and Professor Kiersten Muenchinger of the University of Oregon

We also thank the following returning authors for their valuable contributions and refinements of con-tent in the thirteenth edition: Dr David Mahler of OHSU, Dr John Mitchell of OHSU, and Dr Isabelle Denry of the University of Iowa, previously at The Ohio State University

The organization of the thirteenth edition has been modified extensively to reflect the sequence of content presented to predoctoral dental students at OHSU Chapters are organized by major clinical procedures Chapter 2 presents new content on enamel, dentin, the dentinoenamel junction, and biofilms Chapter

3, another new chapter, describes the concepts of product design and their applications in restorative material selection and treatment design Fundamen-tals of materials science, including the presentation

of physical and mechanical properties, the concepts

of biomechanics, surface chemistry, and optical erties, are consolidated in Chapter 4 Materials test-ing is discussed in extensively revised Chapter 5, which has a greater emphasis on contemporary test-ing methods and standards Chapter 14, new to this edition, is devoted to digital imaging and processing techniques and the materials for those methods All other chapters are reorganized and updated with the most recent science and applications

prop-A website accompanies this textbook Included is the majority of the procedural, or materials handling, content that was in the twelfth edition The website can

be found at http://evolve.elsevier.com/Sakaguchi/restorative/, where you will also find mindmaps of each chapter and extensive text and graphics to sup-plement the print version of the book

Acknowledgments

We are deeply grateful to John Dolan, Executive

Edi-tor at Elsevier, for his guidance in the initial planning

and approval of the project; to Brian Loehr, Senior

Developmental Editor at Elsevier, for his many

sug-gestions and support and prodding throughout the

design process and writing of the manuscript Jodie

Bernard and her team at Lightbox Visuals were

amazing in their ability to create new four-color

images from the original black and white figures

We thank Sara Alsup, Associate Project Manager

at Elsevier, and her team of copyeditors for greatly

improving the style, consistency, and readability of

the text Thanks also to many others at Elsevier for their behind-the-scenes work and contributions to the book

Lastly, we thank our colleagues in our respective institutions for the many informal chats and sugges-tions offered and our families who put up with us being at our computers late in the evenings and on many weekends It truly does take a community to create a work like this textbook and we thank you all

Ronald L Sakaguchi John M Powers

Developments in materials science, robotics, and

biomechanics have dramatically changed the way we

look at the replacement of components of the human

anatomy In the historical record, we find many

approaches to replacing missing tooth structure and

whole teeth The replacement of tooth structure lost to

disease and injury continues to be a large part of

gen-eral dental practice Restorative dental materials are

the foundation for the replacement of tooth structure

Form and function are important considerations

in the replacement of lost tooth structure Although

tooth form and appearance are aspects most easily

recognized, function of the teeth and supporting

tissues contributes greatly to the quality of life The

links between oral and general health are widely

accepted Proper function of the elements of the

oral cavity, including the teeth and soft tissues, is

needed for eating, speaking, swallowing, and proper

breathing

Restorative dental materials make the

reconstruc-tion of the dental hard tissues possible In many areas,

the development of dental materials has progressed

more rapidly than for other anatomical prostheses

Because of their long-term success, patients often

expect dental prostheses to outperform the natural

materials they replace The application of materials

science is unique in dentistry because of the

com-plexity of the oral cavity, which includes bacteria,

high forces, ever changing pH, and a warm, fluid

environment The oral cavity is considered to be the

harshest environment for a material in the body In

addition, when dental materials are placed directly

into tooth cavities as restorative materials, there are

very specific requirements for manipulation of the

material Knowledge of materials science and

biome-chanics is very important when choosing materials

for specific dental applications and when designing

the best solution for restoration of tooth structure

and replacement of teeth

SCOPE OF MATERIALS COVERED IN

RESTORATIVE DENTISTRY

Restorative dental materials include

representa-tives from the broad classes of materials: metals,

polymers, ceramics, and composites Dental

materi-als include such items as resin composites, cements,

glass ionomers, ceramics, noble and base metals,

amalgam alloys, gypsum materials, casting

invest-ments, dental waxes, impression materials, denture

base resins, and other materials used in restorative

procedures The demands for material characteristics

and performance range from high flexibility required

by impression materials to high stiffness required

in crowns and fixed dental prostheses Materials

for dental implants require integration with bone

Some materials are cast to achieve excellent tion to existing tooth structure, whereas others are machined to produce very reproducible dimensions and structured geometries When describing these materials, physical and chemical characteristics are often used as criteria for comparison To understand how a material works, we study its chemical struc-ture, its physical and mechanical characteristics, and how it should be manipulated to produce the best performance

adapta-Most restorative materials are characterized by physical, chemical, and mechanical parameters that are derived from test data Improvements in these characteristics might be attractive in laboratory stud-ies, but the real test is the material’s performance

in the mouth and the ability of the material to be manipulated properly by the dental team In many cases, manipulative errors can negate the techno-logical advances for the material It is therefore very important for the dental team to understand funda-mental materials science and biomechanics to select and manipulate dental materials appropriately

BASIC SCIENCES APPLIED TO RESTORATIVE MATERIALS

The practice of clinical dentistry depends not only

on a complete understanding of the various clinical techniques but also on an appreciation of the funda-mental biological, chemical, and physical principles that support the clinical applications It is important

to understand the ‘how’ and ‘why’ associated with the function of natural and synthetic dental materials

A systems approach to assessing the chemical, physical, and engineering aspects of dental materi-als and oral function along with the physiological, pathological, and other biological studies of the tissues that support the restorative structures pro-vides the best patient outcomes This integrative approach, when combined with the best available scientific evidence, clinician experience, patient preferences, and patient modifiers results in the best patient-centered care

APPLICATION OF VARIOUS

SCIENCES

In the chapters that follow, fundamental teristics of materials are presented along with numer-ous practical examples of how the basic principles relate to clinical applications Test procedures and techniques of manipulation are discussed briefly but not emphasized Many of the details of manipulation have been moved to the book’s website at http://evolve.elsevier.com/sakaguchi/restorative

A more complete understanding of fundamental

principles of materials and mechanics is important

for the clinician to design and provide a prognosis for

restorations For example, the prognosis of long-span

fixed dental prostheses, or bridges, is dependent on

the stiffness and elasticity of the materials When

considering esthetics, the hardness of the material

is an important property because it influences the

ability to polish the material Some materials release

fluoride when exposed to water, which might be

ben-eficial in high-caries-risk patients When selecting a

ceramic for in-office fabrication of an all-ceramic

crown, the machining characteristic of ceramics is

important Implants have a range of bone and soft

tissue adaptation that are dependent on surface

tex-ture, coatings, and implant geometry These are just

a few examples of the many interactions between the

clinical performance of dental materials and

funda-mental scientific principles

The toxicity of and tissue reactions to dental

mate-rials are receiving more attention as a wider variety

of materials are being used and as federal agencies

demonstrate more concern in this area A further

indication of the importance of the interaction of

materials and tissues is the development of

recom-mended standard practices and tests for the

biologi-cal interaction of materials through the auspices of

the American Dental Association (ADA)

After many centuries of dental practice, we

con-tinue to be confronted with the problem of replacing

tooth tissue lost by either accident or disease In an

effort to constantly improve our restorative

capa-bilities, the dental profession will continue to draw

from materials science, product design, engineering,

biology, chemistry, and the arts to further develop an

integrated practice of dentistry

FUTURE DEVELOPMENTS IN

BIOMATERIALS

In the United States over 60% of adults aged 35 to

44 have lost at least one permanent tooth to an

acci-dent, gum disease, a failed root canal, or tooth decay

In the 64- to 65-year-old category, 25% of adults have

lost all of their natural teeth For children aged 6 to 8,

26% have untreated dental caries, and 50% have been

treated for dental decay The demand for restorative

care is tremendous Advances in endodontology and

periodontology enable people to retain teeth longer,

shifting restorative care from replacement of teeth

to long-term restoration and maintenance

Develop-ment of successful implant therapies has encouraged

patients to replace individual teeth with fixed, single

tooth restorations rather than with fixed or

remov-able dental prostheses For those patients with good

access to dental care, single tooth replacements with

implants are becoming a more popular option because they do not involve the preparation of adjacent teeth

as for a fixed, multi-unit restoration Research into implant coatings, surface textures, graded proper-ties, alternative materials, and new geometries will continue to grow For those with less adequate access, removable prostheses will continue to be used

An emphasis on esthetics continues to be lar among consumers, and this will continue to drive the development of tooth whitening systems and esthetic restorations There appears to be an emerg-ing trend for a more natural looking appearance with some individuality as opposed to the uniform, spar-kling white dentition that was previously requested

popu-by many patients This will encourage ers to develop materials that mimic natural dentition even more closely by providing the same depth of color and optical characteristics of natural teeth.With the aging of the population, restorations for exposed root surfaces and worn dentitions will become more common These materials will need to function in an environment with reduced salivary flow and atypical salivary pH and chemistry Adhe-sion to these surfaces will be more challenging This segment of the population will be managing multi-ple chronic diseases with many medications and will have difficulty maintaining an adequate regimen of oral home care Restorative materials will be chal-lenged in this difficult environment

manufactur-The interaction between the fields of biomaterials and molecular biology is growing rapidly Advances

in tissue regeneration will accelerate The ments in nanotechnology will soon have a major impact on materials science The properties we cur-rently understand at the macro and micro levels will

develop-be very different at the nano level Biofabrication and bioprinting methods are creating new structures and materials This is a very exciting time for materials research and clinicians will have much to look for-ward to in the near future as this body of research develops new materials for clinical applications

Bibliography

American Association of Oral and Maxillofacial Surgeons:

Dental implants http://www.aaoms.org/dental_implants.php Accessed August 28, 2011

Centers for Disease Control and Prevention: National Health and Nutrition Examination Study http://www.cdc.gov/nchs/nhanes/nhanes2005-2006/nhanes05_06 htm Accessed August 28, 2011

Choi CK, Breckenridge MT, Chen CS: Engineered materials and the cellular microenvironment: a strengthening interface between cell biology and bioengineering,

Trends Cell Biol 20(12):705, 2010

Horowitz RA, Coelho PG: Endosseus implant: the journey

and the future, Compend Contin Educ Dent 31(7):545,

2010

Jones JR, Boccaccini AR: Editorial: a forecast of the future

for biomaterials, J Mater Sci: Mater Med 17:963, 2006.

Kohn DH: Current and future research trends in dental

biomaterials, Biomat Forum 19(1):23, 1997.

Nakamura M, Iwanaga S, Henmi C, et al: Biomatrices and

biomaterials for future developments of bioprinting and

biofabrication, Biofabrication 2(1):014110, 2010 Mar 10

Epub

National Center for Chronic Disease Prevention and Health

Promotion (CDC): Oral health, preventing cavities, gum

disease, tooth loss, and oral cancers, at a glance, 2010

National Institute of Dental Research: National Institutes of

Health (NIH): International state-of-the-art conference on

restorative dental materials, Bethesda, MD, Sept 8-10,

1986, NIH

National Institute of Dental and Craniofacial Research:

A plan to eliminate craniofacial, oral, and dental health parities, 2002 http://www.nidcr.nih.gov/NR/rdonlyres/54B65018-D3FE-4459-86DD-AAA0AD51C82B/0/ hdplan.pdf

dis-Oregon Department of Human Services, Public Health Division: The burden of oral disease in Oregon, Nov, 2006

U.S Department of Health and Human Services: Oral health

in America: a report of the Surgeon General—executive summary, Rockville, MD, 2000, U.S Department of Health and Human Services, National Institute of Dental and Craniofacial Research, National Institutes of Health

The Dentin-Enamel Junction

Oral Biofilms and Restorative Dental Materials

The tooth contains three specialized calcified

tissues: enamel, dentin, and cementum (Figure 2-1)

Enamel is unique in that it is the most highly

calci-fied tissue in the body and contains the least organic

content of any of these tissues Enamel provides

the hard outer covering of the crown that allows

efficient mastication Dentin and cementum, like

bone, are vital, hydrated, biological composite

structures formed mainly from a collagen type I

matrix reinforced with the calcium phosphate

min-eral called apatite Dentin forms the bulk of the tooth

and is joined to the enamel at the dentin-enamel

junction (DEJ) The dentin of the tooth root is covered

by cementum that provides connection of the tooth

to the alveolar bone via the periodontal ligament

Although the structure of these tissues is often

described in dental texts, the properties are often

discussed only superficially However, these

proper-ties are important in regard to the interrelationships

of the factors that contribute to the performance

necessary for the optimum function of these tissues

In restorative dentistry we are interested in

pro-viding preventive treatments that will maintain

tissue integrity and replace damaged tissues with

materials that ideally will mimic the natural

appear-ance and performappear-ance of those tissues when

neces-sary Thus knowledge of the structure and properties

of these tissues is desirable both as a yardstick to

measure the properties and performance of

restor-ative materials and as a guide to the development

of materials that will mimic their structure and

func-tion In addition, many applications, such as dental

bonding, require us to attach synthetic materials to

the calcified tissues, and these procedures rely on detailed knowledge of the structure and properties

of the adhesive tissue substrates

ENAMEL

Figure 2-1 shows a schematic diagram of a rior tooth sectioned to reveal the enamel and dentin components Enamel forms the hard outer shell of the crown and as the most highly calcified tissue is well suited to resisting wear due to mastication.Enamel is formed by ameloblasts starting at the dentin-enamel junction (DEJ) and proceeding out-ward to the tooth surface The ameloblasts exchange signals with odontoblasts located on the other side

poste-of the DEJ at the start poste-of the enamel and dentin mation, and the odontoblasts move inward from the DEJ as the ameloblasts forming enamel move out-ward to form the enamel of the crown Most of the enamel organic matrix composed of amelogenins and enamelins is resorbed during tooth maturation

for-to leave a calcified tissue that is largely composed of mineral and a sparse organic matrix The structural arrangement of enamel forms keyhole-shaped struc-

tures known as enamel prisms or rods that are about

5 μm across as seen in Figure 2-2.The overall composition is about 96% mineral by weight, with 1% lipid and protein and the remainder being water The organic portion and water probably play important roles in tooth function and pathology, and it is often more useful to describe the composition

on a volume basis On that basis we see the organic components make up about 3% and water 12% of the structure The mineral is formed and grows into very long crystals of hexagonal shape about 40 nm across; these have not been synthetically duplicated There is some evidence that the crystals may span the whole enamel thickness, but this is difficult to prove because most preparation procedures lead to frac-ture of the individual crystallites It appears that they are at least thousands of nanometers long If this is true, then enamel crystals provide an extraordinary

“aspect” ratio (length to width ratio) for a nanoscale material, and they are very different from the much smaller dentin crystals The crystals are packed into enamel prisms or rods that are about 5 μm across as shown in Figure 2-2 These prisms are revealed easily

by acid etching and extend in a closely packed array from the DEJ to the enamel surface and lie roughly perpendicular to the DEJ, except in cuspal areas

where the rods twist and cross, known as decussation,

which may increase fracture resistance About 100 crystals of the mineral are needed to span the diam-eter of a prism, and the long axes of the crystals tend

to align themselves along the prism axes, as seen in

Figure 2-2

Enamel Dentin

Pulp

Innercervical

Outer

Inner

FIGURE 2.1 Schematic diagram of a tooth cut

longitudi-nally to expose the enamel, dentin, and the pulp chamber.

On the right side are illustrations of dentin tubules as

viewed from the top, which shows the variation in the

tubule number with location At the left is an illustration of

the change in direction of the primary dentin tubules as

secondary dentin is formed (From Marshall SJ, et al: Acta

Mater 46, 2529-2539, 1998.)

The crystals near the periphery of each prism

deviate somewhat from the long axis toward the

interface between prisms The deviation in the tail

of the prism is even greater The individual crystals

within a prism are also coated with a thin layer of

lipid and/or protein that plays important roles in

mineralization, although much still remains to be

learned about the details Recent work suggests that

this protein coat may lead to increased toughness of

the enamel The interfaces between prisms, or

inter-rod enamel, contain the main organic components

of the structure and act as passageways for water

and ionic movement These areas are also known as

prism sheaths These regions are of vital importance

in etching processes associated with bonding and

other demineralization processes, such as caries

Etching of enamel with acids such as phosphoric

acid, commonly used in enamel bonding, eliminates

smear layers associated with cavity preparation,

dissolves persisting layers of prismless enamel in deciduous teeth, and differentially dissolves enamel crystals in each prism The pattern of etched enamel

is categorized as type 1 (preferential prism core ing, Figure 2-2, A); type 2 (preferential prism periph-ery etching, Figure 2-3, C), and type 3 (mixed or

etch-uniform) Sometimes these patterns appear side by side on the same tooth surface (Figure 2-3, E) No

differences in micromechanical bond strength of the different etching patterns have been established In a standard cavity preparation for a composite, the ori-entation of the enamel surfaces being etched could

be perpendicular to enamel prisms (perimeter of the cavity outline), oblique cross section of the prisms (beveled occlusal or proximal margins), and axial walls of the prisms (cavity preparation walls) Dur-ing the early stages of etching, when only a small amount of enamel crystal dissolution occurs, it may

be difficult or impossible to detect the extent of the

InterrodenamelHeadTail

FIGURE 2.2 Enamel microstructure showing a schematic diagram of keyhole-shaped enamel prisms or rods about 5 μm

in diameter (B) Atomic force microscopy (AFM) images showing prism cross sections in A and along axes of the prisms

in C Crystallite orientation deviates in the inter-rod and tail area, and the organic content increases in the inter-rod area

(Modified from Habelitz S, et al: Arch Oral Biol 46, 173-183, 2001.)

process However, as the etching pattern begins to

develop, the surface etched with phosphoric acid

develops a frosty appearance (Figure 2-3, B), which

has been used as the traditional clinical indicator for

sufficient etching This roughened surface provides

the substrate for infiltration of bonding agents that

can be polymerized after penetration of the etched

enamel structure so that they form micromechanical bonds to the enamel when polymerized With self-etching bonding agents, this frosty appearance can-not be detected

There are two other important structural tions of enamel Near the DEJ the enamel prism structure is not as well developed in the very first

D , Bonding agent revealed after dissolving enamel E, Mixed etch patterns showing type 1 (light prisms with dark periphery)

and type 2 (dark cores with light periphery) etching on same surface after Marshall et al, 1975 JDR Marshall GW, Olson

LM, Lee CV: SEM Investigation of the variability of enamel surfaces after simulated clinical acid etching for pit and fissure sealants, J Dent Res 54:1222–1231, 1975 Part C from Marshall, Olson and Lee, JDR 1975 (same as above) and Part E from Marshall, Marshall and Bayne, 1988: Marshall GW, Marshall SJ, Bayne SC: Restorative dental materials: scanning electron microscopy and x-ray microanalysis, Scanning Microsc 2:2007–2028, 1988

enamel formed, so that the enamel very close to the

DEJ may appear aprismatic or without the prism

like structure Similarly, on the outer surface of the

enamel, at completion of the enamel surface, the

ameloblasts degenerate and leave a featureless layer,

called prismless enamel, on the outer surface of the

crown This layer is more often observed in

decidu-ous teeth and is often worn off in permanent teeth

However, if present, this causes some difficulty in

getting an effective etching pattern and may require

roughening of the surface or additional etching

treat-ments The outer surface of the enamel is of great

clinical significance because it is the surface

sub-jected to daily wear and undergoes repeated cycles of

demineralization and remineralization As a result of

these cycles, the composition of the enamel crystals

may change, for example, as a result of exposure to

fluoride Thus the properties of the enamel might be

expected to vary from the external to the internal

sur-face Such variations, including a thin surface veneer

of fluoride-rich apatite crystals, create differences in

the enamel properties within the enamel Enamel is

usually harder at the occlusal and cuspal areas and

less hard nearer the DEJ Figure 2-4 shows an

exam-ple of the difference in hardness

THE MINERAL

The mineral of all calcified tissues is a highly

defective relative of the mineral hydroxyapatite,

or HA The biological apatites of calcified tissues

are different than the ideal HA structure in that the

defects and chemical substitutions generally make it

weaker and more soluble in acids Hydroxyapatite

has the simple formula Ca10(PO4)6(OH)2, with an

ideal molar ratio of calcium to phosphorus (Ca/P) of

1.67 and a hexagonal crystal structure The apatite of

enamel and dentin has a much more variable sition that depends on its formative history and other chemical exposures during maturity Thus the min-eral in enamel and dentin is a calcium-deficient, car-bonate-rich, and highly substituted form related to

compo-HA Metal ions such as magnesium (Mg) and sodium (Na) may substitute for calcium, whereas carbonate substitutes for the phosphate and hydroxyl groups These substitutions distort the structure and make it more soluble Perhaps the most beneficial substitu-tion is the fluorine (F) ion, which substitutes for the hydroxyl group (OH) in the formula and makes the structure stronger and less soluble Complete substi-tution of F for (OH) in hydroxyapatite yields fluo-roapatite mineral, Ca10(PO4)6(F)2, that is much less soluble than HA or the defective apatite of calcified tissues It is worth noting that HA has attracted con-siderable attention as an implantable calcified tissue replacement It has the advantage of being a purified and stronger form of the natural mineral and releases

no harmful agents during biological degradation Its major shortcoming is that it is extremely brittle and sensitive to porosity or defects and therefore frac-tures easily in load-bearing applications

The approximate carbonate contents of the enamel and dentin apatites are significantly different, about 3% and 5% carbonate, respectively All other factors being equal, this would make the dentin apatite more soluble in acids than enamel apatite Things are not equal, however, and the dentin apatite crystals are much smaller than the enamel crystals This means that the dentin crystals present a higher surface area

to attacking acids and contain many more defects per unit volume and thus exhibit considerably higher solubility Finally, as discussed further below, the dentin mineral occupies only about 50% of the den-tin structure, so there is not as much apatite in the dentin as there is in enamel All of these factors mul-tiply the susceptibility of dentin to acid attack and provide insight into the rapid spread of caries when

it penetrates the DEJ

DENTIN

Dentin is a complex hydrated biological ite structure that forms the bulk of the tooth Fur-thermore, dentin is modified by physiological, aging, and disease processes that result in different forms

compos-of dentin These altered forms compos-of dentin may be the precise forms that are most important in restorative dentistry Some of the recognized variations include primary, secondary, reparative or tertiary, sclerotic, transparent, carious, demineralized, remineralized, and hypermineralized These terms reflect altera-tions in the fundamental components of the struc-ture as defined by changes in their arrangement,

Buccal

Hardness (GPa)

Lingual

65.554.543.532.5

B cca

Buccaalal

H

Hardard

FIGURE 2.4 Nanoindentation mapping of the

mechani-cal properties of human molar tooth enamel. (From Cuy JL,

et al: Arch Oral Biol 47(4), 281-291, 2002.)

interrelationships, or chemistry A number of these

may have important implications for our ability to

develop long-lasting adhesion or bonds to dentin

Primary dentin is formed during tooth

develop-ment Its volume and conformation, reflecting tooth

form, vary with the size and shape of the tooth

Den-tin is composed of about 50 volume percent (vol%)

carbonate-rich, calcium-deficient apatite; 30 vol%

organic matter, which is largely type I collagen; and

about 20 vol% fluid, which is similar to plasma Other

noncollagenous proteins are thought to be involved

in dentin mineralization and other functions such as

controlling crystallite size and orientation; however, these functions are not discussed further in this text The major components are distributed into distinc-tive morphological features to form a vital and com-plex hydrated composite in which the morphology varies with location and undergoes alterations with age or disease

The tubules, one distinct and important feature

of dentin, represent the tracks taken by the blastic cells from the DEJ or cementum at the root to the pulp chamber and appear as tunnels piercing the dentin structure (Figure 2-5) The tubules converge

odonto-on the pulp chamber, and therefore tubule density and orientation vary from location to location (see

Figure 2-1) Tubule number density is lowest at the DEJ and highest at the predentin surface at the junc-tion to the pulp chamber, where the odontoblastic cell bodies lie in nearly a close-packed array Lower tubule densities are found in the root The contents

of the tubules include odontoblast processes, for all

or part of their course, and fluid The extent of the odontoblast process is still uncertain, but evidence is mounting that it extends to the DEJ For most of its course, the tubule lumen is lined by a highly min-eralized cuff of peritubular dentin roughly 0.5 to 1

μm thick (Figure 2-6) Because the peritubular tin forms after the tubule lumen has been formed, some argue that it may be more properly termed

den-intratubular dentin and contains mostly apatite tals with little organic matrix A number of studies have concluded that the peritubular dentin does not contain collagen, and therefore might be considered

crys-a sepcrys-arcrys-ate ccrys-alcified tissue The tubules crys-are sepcrys-arcrys-ated

by intertubular dentin composed of a matrix of type

I collagen reinforced by apatite (see Figures 2-5 and 2-6) This arrangement means that the amount of intertubular dentin varies with location The apatite

FIGURE 2.6 Fracture surface of the dentin viewed from the occlusal in A and longitudinally in B Peritubular

(P) (also called intratubular) dentin forms a cuff or lining around each tubule The tubules are separated from one another

by intertubular dentin (I) (Courtesy of G W Marshall.)

30kv 2.00kx 5.0 959

FIGURE 2.5 Scanning electron microscopy (SEM) image

of normal dentin showing its unique structure as seen

from two directions. At the top is a view of the tubules,

each of which is surrounded by peritubular dentin Tubules

lie between the dentin-enamel junction (DEJ) and converge

on the pulp chamber The perpendicular surface at the

bottom shows a fracture surface revealing some of the

tubules as they form tunnel-like pathways toward the pulp

The tubule lumen normally contains fluid and processes of

the odontoblastic cells (From Marshall GW: Quintessence Int

24, 606-617, 1993.)

crystals are much smaller (approximately 5 × 30 ×

100 nm) than the apatite found in enamel and

con-tain 4% to 5% carbonate The small crystallite size,

defect structure, and higher carbonate content lead

to the greater dissolution susceptibility described

above

Estimates of the size of tubules, the thickness of

the peritubular region, and the amount of

intertubu-lar dentin have been made in a number of studies

Calculations for occlusal dentin as a function of

posi-tion from these data show the percent tubule area

and diameter vary from about 22% and 2.5 μm near

the pulp to 1% and 0.8 μm at the DEJ Intertubular

matrix area varies from 12% at the predentin to 96%

near the DEJ, whereas peritubular dentin ranges

from over 60% down to 3% at the DEJ Tubule

den-sities are compared in Table 2-1 based on work by

various investigators It is clear that the structural

components will vary considerably over their course,

and necessarily result in location-dependent

varia-tions in morphology, distribution of the structural

elements, and important properties such as

perme-ability, moisture content, and available surface area

for bonding and may also affect bond strength,

hard-ness, and other properties

Because the odontoblasts come to rest just inside

the dentin and line the walls of the pulp chamber

after tooth formation, the dentin-pulp complex can

be considered a vital tissue This is different than

mature enamel Over time secondary dentin forms

and the pulp chamber gradually becomes smaller

The border between primary and secondary dentin

is usually marked by a change in orientation of the

dentin tubules Furthermore, the odontoblasts react

to form tertiary dentin in response to insults such as

caries or tooth preparation, and this form of dentin is

often less well organized than the primary or

second-ary dentin

Early enamel carious lesions may be reversed

by remineralization treatments However, effective

re mineralization treatments are not yet available for

dentin and therefore the current standard of care

dictates surgical intervention to remove highly

dam-aged tissue and then restoration as needed Thus it is

important to understand altered forms of dentin and the effects of such clinical interventions

When dentin is cut or abraded by dental ments, a smear layer develops and covers the surface and obscures the underlying structure (Figure 2-7) The bur cutting marks are shown in Figure 2-7, A,

instru-and at higher magnification in Figure 2-7, B Figure 2-7, C, shows the smear layer thickness from the side

and the development of smear plugs as the cut tin debris is pushed into the dentin tubule lumen The advantages and disadvantages of the smear layer have been extensively discussed for several decades It reduces permeability and therefore aids

den-in maden-intaden-inden-ing a drier field and reduces den-infiltration

of noxious agents into the tubules and perhaps the pulp However, it is now generally accepted that it

is a hindrance to dentin bonding procedures and, therefore, is normally removed or modified by some form of acid conditioning

Acid etching or conditioning allows for removal

of the smear layer and alteration of the superficial dentin, opening channels for infiltration by bonding agents Figure 2-8 shows what happens in such an etching treatment The tubule lumens widen as the peritubular dentin is preferentially removed because

it is mostly mineral with sparse protein The widened lumens form a funnel shape that is not very retentive

Figure 2-9 shows these effects in a slightly ent way Unetched dentin in Figure 2-9, A, has small

differ-tubules and peritubular dentin, which is removed in the treated dentin at the exposed surface after etching

(bottom) The two-dimensional network of collagen

type I fibers is shown after treatment in Figure 2-9, A

Figure 2-9, B, shows progressive demineralization of

a dentin collagen fibril in which the external mineral and proteins are slowly removed to reveal the typi-cal banded pattern of type I collagen In Figure 2-9,

C, this pattern is seen at high magnification of the treated dentin in Figure 2-9, A.

If the demineralized dentin is dried, the ing dentin matrix shrinks and the collagen fibrils become matted and difficult to penetrate by bonding agents This is shown in Figure 2-10, which compares demineralized and dried dentin with demineralized and hydrated dentin

remain-Most restorative procedures involve dentin that has been altered in some way Common alterations include formation of carious lesions that form vari-ous zones and include transparent dentin that forms under the caries infected dentin layer Transparent dentin results when the dentin tubules become filled with mineral, which changes the refractive index of the tubules and produces a translucent or transpar-ent zone

Figure 2-11 shows a section through a tooth with

a carious lesion, which has been stained to reveal its zones The gray zone under the stained and severely

TABLE 2.1 Comparison of Mean Numerical Density

of Tubules in Occlusal Dentin*

Outer Dentin Middle Dentin Inner Dentin

demineralized dentin is the transparent layer (Figure

2-11, A) Figure 2-11, B, shows the transparent dentin

in which most of the tubule lumens are filled with

mineral After etching, as shown in Figure 2-11, C the

peritubular dentin is etched away, but the tubules

retain plugs of the precipitated mineral, which is

more resistant to etching This resistance to etching

makes bonding more difficult

Several other forms of transparent dentin are

formed as a result of different processes A second

form of transparent dentin results from bruxism

An additional form of transparent dentin results

from aging as the root dentin gradually becomes

transparent In addition noncarious cervical lesions

(NCCLs), often called abfraction or notch lesions, form

at the enamel-cementum or enamel-dentin junction,

usually on facial or buccal surfaces Their etiology

is not clear at this point; their formation has been

attributed to abrasion, tooth flexure, and erosion or

some combination of these processes Nonetheless

these lesions occur with increasing frequency with

age, and the exposed dentin becomes transparent as

the tubules are filled Figure 2-12 shows examples of

transparent dentin in which the tubule lumens are completely filled

The properties of the transparent dentin may fer from one to another depending on the processes that lead to deposit of the mineral in the tubules Several studies have shown that elastic properties

dif-of the intertubular dentin are not altered by aging, although the structure may become more suscepti-ble to fracture Similarly, arrested caries will contain

transparent dentin and this has often been called rotic dentin, a term that implies it may be harder than normal dentin However, other studies have shown that the elastic properties of the intertubular den-tin may actually be unaltered or lower than normal dentin

scle-Physical and Mechanical Properties

The marked variations in the structural elements

of dentin when located within the tooth imply that the properties of dentin will vary considerably with location That is, variable structure leads to variable properties

A

FIGURE 2.7 Smear layer formation A, Bur marks on dentin preparation B, Higher magnification showing smear layer

surface and cutting debris C, Section showing smear layer (SL) and smear plugs (S.P.) (A and B from Marshall GW, et al: Scanning Microsc 2, 2007-2028, 1988; C from Pashley DH, et al: Arch Oral Biol 33, 265-270, 1988.)

Because one major function of tooth structure is

to resist deformation without fracture, it is useful to

have knowledge of the forces that are experienced by

teeth during mastication Measurements have given

values on cusp tips of about 77 kg distributed over

the cusp tip area of 0.039 cm2 , suggesting a stress of

about 200 MPa

Difficulties in Testing

In Table 2-2, values are presented for some

impor-tant properties of enamel and dentin The wide

spread of values reported in the literature is

remark-able Some of the reasons for these discrepancies

should be appreciated and considered in practice or

when reading the literature

First, human teeth are small, and therefore it is

dif-ficult to get large specimens and hold them in such a

way that you can measure properties This makes the

use of standard mechanical testing such as tensile,

compressive, or shear tests difficult When testing

bonded teeth, the problem is even more complicated,

and special tests have been developed to obtain insights into these properties From the previous dis-cussion of structural variations, it is also clear that testing such small inhomogeneous specimens means that the properties will not be uniform

Another problem is the great variation in ture in both tissues Enamel prisms are aligned generally perpendicular to the DEJ, whereas dentin tubules change their number density with depth as they course toward the pulp chamber Preparing a uniform sample with the structures running all in one direction for testing is challenging In addition, properties generally vary with direction and location and the material is not isotropic; therefore, the best

struc-a single vstruc-alue cstruc-an tell you is some struc-averstruc-age vstruc-alue for the material

Storage and time elapsed since extraction are also important considerations Properties that exist in a

natural situation or in situ or in vivo are of greatest

interest Clearly this condition is almost impossible

to achieve in most routine testing, so changes that

FIGURE 2.8 Stages of dentin demineralization A, Schematic showing progressive stages of dentin demineralization

B to D, Atomic force microscopy (AFM) images showing stages of etching The etching leads to wider lumens as

peritu-bular dentin is dissolved and funnel-shaped openings are formed (AFM images from Marshall GW: Quintessence Int 24, 606-617, 1993.)

FIGURE 2.9 Etching of dentin removes mineral from the intertubular dentin matrix leaving a collagen-rich layer and widening the dentin tubule orifices A, After etching the tubule lumens are enlarged and the collagen network surround- ing the tubules can be seen after further treatment B, Isolated dentin collagen fiber is slowly demineralized revealing the

typical 67 nm repeat pattern of type I collagen C, High magnification view of collagen fibers in A (A and C from Marshall

GW, et al: Surface Science 491, 444-455, 2001; B modified from Balooch M, et al: J Struct Biol 162, 404-410, 2008.)

FIGURE 2.10 Demineralized dentin is sensitive to moisture and shrinks on drying A, Demineralized dentin undergoes

shrinkage when air dried forming a collapsed layer of collagen that is difficult to infiltrate with resin bonding agents

B, When kept moist, the collagen network is open and can be penetrated by bonding agents (From Marshall GW, et al:

J Dent 25, 441-458, 1997.)

have occurred as a result of storage conditions prior

to testing must be considered It is also important to

consider biological hazards because extracted teeth

must be treated as potentially infective How do you

sterilize the teeth without altering their properties?

Autoclaving undoubtedly alters the properties of

proteins, and is therefore not appropriate for dentin,

and might also affect enamel

Finally, the fluid content of these tissues must be

considered Moisture is a vital component of both

tissues and in vivo conditions cannot be replicated

if the tissues have been desiccated (see Figure 2-10)

This becomes a critically important consideration in

bonding to these tissues, as is discussed further in

Chapter 13 In contrast to the importance of this issue

is the issue of convenience It is much more difficult

to test the tissues in a fully hydrated condition than

in a dry condition All of these factors and a number

of others, such as temperature of testing, will ence the results and contribute to a spread in the val-ues reported for the properties

influ-Despite these limitations, some generalizations about the properties of these tissues are useful (see

Table 2-1) Root dentin is generally weaker and softer than coronal dentin Enamel also appears to vary in its properties, with cuspal enamel being stronger and harder than other areas, presumably as an adaption

to masticatory forces Dentin is less stiff than enamel (i.e., has a lower elastic modulus), and has a higher fracture toughness This may be counterintuitive but

A

Trans

B

10203040

C

10203040

FIGURE 2.11 Transparent dentin associated with carious lesions A, Carious lesion showing dentin carious zones revealed by staining, including the grayish transparent zone B, Atomic force microscopy (AFM) of carious transparent dentin before etching C, After etching the tubule lumens remain filled even as the peritubular dentin is etched away

(A from Zheng L, et al: Eur J Oral Sci 111, 243-252, 2003; B and C from Marshall GW, et al: Dent Mater 17, 45-52, 2001b.)

will become clearer when we define these terms in

Chapter 4 In addition, dentin is viscoelastic, which

means that its mechanical deformation

characteris-tics are time dependent, and elastic recovery is not

instantaneous Thus dentin may be sensitive to how

rapidly it is strained, a phenomenon called strain

rate sensitivity Strain rate sensitivity is characteristic

of polymeric materials; the collagen matrix imparts

this property to tissues such as dentin Under normal

circumstances, enamel and other ceramic materials

do not show this characteristic in their mechanical

properties

The Dentin-Enamel Junction

The dentin-enamel junction (DEJ) is much more

than the boundary between enamel and dentin

Because enamel is very hard and dentin is much

softer and tougher, they need to be joined together

to provide a biomechanically compatible system Joining such dissimilar materials is a challenge, and

it is not completely clear how nature has plished this However, the DEJ not only joins these two tissues but also appears to resist cracks in the enamel from penetrating into dentin and leading to tooth fracture as shown in Figure 2-13, A Many such

accom-cracks exist in the enamel but do not seem to gate into the dentin If the DEJ is intact, it is unusual

propa-to have propa-tooth fracture except in the face of severe trauma In Figure 2-13, B, microhardness inden-

tations have been placed to drive cracks toward

the DEJ (orange) The crack stops at or just past the

interface This image also shows that the DEJ is loped with its concavity directed toward the enamel This means that most cracks approach the DEJ at

scal-an scal-angle, scal-and this may lead to arrest of mscal-any of the cracks The scalloped structure actually has three lev-els: scallops, microscallops within the scallops, and

a finer structure Figures 2-13, C, and 2-13, D, show

images of larger scallops in molars (~24 μm across) and smaller scallops (~15 μm across) in anterior teeth after the removal of the enamel Finite element mod-els suggest that the scallops reduce stress concentra-tions at the interface, but it is not known whether the larger scallop size in posterior teeth is an adap-tion to higher masticatory loads or a developmental variation In Figure 2-13, E, the crystals of dentin are

almost in contact with those of the enamel, so that the anatomical DEJ is said to be optically thin How-ever, measurements of property variations across the DEJ show that this is a graded interface with proper-ties varying from those of the enamel to the adjacent mantle dentin over a considerable distance This gra-dient, which is due in part to the scalloped nature of the DEJ, makes the functional width of the DEJ much larger than its anatomical appearance and further

15kv 2.0kx 5.00 523

FIGURE 2.12 Transparent dentin As seen from the facial, A, and longitudinal, B, directions The transparent dentin results

from filling of the tubules with mineral deposits that alter the optical properties of the tooth (Courtesy of Marshall GW.)

TABLE 2.2 Properties of Enamel and Dentin

Density 2.96 g/cm3 2.1 g/cm3

Compressive

Modulus of elasticity 60-120 GPa 18-24 GPa

Proportional limit 70-353 MPa 100-190 MPa

Strength 94-450 MPa 230-370 MPa

Tensile

Modulus of elasticity 11-19 GPa

Strength 8-35 MPa 30-65 MPa

Shear strength 90 MPa 138 MPa

Flexural strength 60-90 MPa 245-280 MPa

FIGURE 2.13 Cracks in enamel appear to stop at the dentin-enamel junction (DEJ) A, Low-magnification view of

cracks in enamel B, Indentation-generated cracks stop near or at the scalloped DEJ (orange) C, Large scallops in molars

D , Smaller scallops in anterior teeth E, Crystals of the enamel are nearly in contact with dentin crystals at the DEJ forming

an optically thin but functionally wide union (A, C-E from Marshall SJ, et al: J European Ceram Soc 23, 2897-2904, 2003;

B from Imbeni V, et al: Nature Mater 4, 229-232, 2005.)

reduces stresses In addition, although collagen is

absent from enamel, collagen fibers cross the DEJ

from dentin into enamel to further integrate the two

tissues At this point, no unique components, such as

proteins, have been identified that could serve as a

special adhesive that bonds the enamel to the dentin

ORAL BIOFILMS AND RESTORATIVE

DENTAL MATERIALS

Biofilms are complex, surface-adherent, spatially

organized polymicrobial communities containing

bacteria surrounded by a polysaccharide matrix

Oral biofilms that form on the surfaces of teeth and

biomaterials in the oral cavity are also known as

den-tal plaque When the human diet is rich in fermentable

carbohydrates, the most prevalent organisms shown

to be present in dental plaque are adherent

acido-genic and aciduric bacteria such as streptococci and

lactobacilli that are primarily responsible for dental

caries Other consequences of long-term oral biofilm

accumulation can also include periodontal diseases

and peri-implantitis (inflammation of the soft and

hard tissues surrounding an implant), depending on

the location of attachment of the biofilm

Biofilm formation on hard surfaces in the oral

cavity is a sequential process A conditioning film

from saliva (known as pellicle) containing adsorbed

macromolecules such as phosphoproteins and

gly-coproteins is deposited on tooth structure and

bio-materials within minutes after a thorough cleaning

This stage is followed by the attachment of

plank-tonic (free-floating) bacteria to the pellicle Division

of the attached initial colonizing bacterial species

produces microcolonies, and subsequent attachment

of later colonizing species results in the formation of

matrix-embedded multispecies biofilms These

bio-films can mature over time if they are not detached

by mechanical removal or intrinsic factors

Biofilm formation occurs via complicated physical

and cellular interactions between the substrate,

pel-licle, and bacteria These interactions occur at several

levels and can include physical proximity, metabolic

exchange, signal molecule-mediated

communica-tion, exchange of genetic material, production of

inhibitory factors, and co-aggregation (“specific

cell-to-cell recognition between genetically distinct cell

types,” as defined by Kolenbrander et al., 2006)

The pellicle contains a variety of receptor

mol-ecules that are recognized primarily by streptococci

(Figure 2-14) This is evident in healthy

individu-als, who typically have biofilms containing a thin

layer of adherent gram-positive cocci The ability to

bind to nonshedding surfaces such as enamel gives

streptococci a tremendous advantage and is

consis-tent with the observation that streptococci constitute

60% to 90% of the initial bacterial flora on enamel in situ Furthermore, the streptococci are less sensitive

to exposure to air than most oral bacteria because they are facultatively anaerobic and can participate

in modifying the biofilm environment to a more reduced state, a condition often considered to favor

an ecological shift towards gram-negative anaerobes.Interactions among human oral bacteria are piv-otal to the development of oral biofilms (see Figure 2-14) In the first 4 hours of biofilm formation, gram-positive cocci appear to predominate, particularly

FIGURE 2.14 Spatiotemporal model of oral bacterial colonization, showing recognition of salivary pellicle receptors by early colonizing bacteria and co- aggregations between early colonizers, fusobacteria, and late coloniz- ers of the tooth surface. Starting at the bottom, primary

colonizers bind via adhesins (round-tipped black line symbols)

to complementary salivary receptors (blue-green vertical round-topped columns) in the acquired pellicle coating the tooth surface Secondary colonizers bind to previously bound bacteria Sequential binding results in the appear-ance of nascent surfaces that bridge with the next co- aggregating partner cell The bacterial strains shown are

Actinobacillus actinomycetemcomitans, Actinomyces israelii, Actinomyces naeslundii, Capnocytophaga gingivalis, Capnocyto- phaga ochracea, Capnocytophaga sputigena, Eikenella corrodens, Eubacterium spp., Fusobacterium nucleatum, Haemophilus parainfluenzae, Porphyromonas gingivalis, Prevotella denticola, Prevotella intermedia, Prevotella loescheii, Propionibacterium acnes, Selenomonas flueggei, Streptococcus gordonii, Streptococ- cus mitis, Streptococcus oralis, Streptococcus sanguis , Treponema spp., and Veillonella atypica (From Kolenbrander PE, et al: Microbiol Mol Biol Rev 66, 486-505, 2002.)

mitis group streptococci After 8 hours of growth, the

majority of the bacterial population continues to be

largely coccoid, but rod-shaped organisms are also

observed By 24 to 48 hours, thick deposits of cells

with various morphologies can be detected,

includ-ing coccoid, coccobacilliary, rod-shaped, and

fila-mentous bacteria Within 4 days of biofilm growth,

an increase in the numbers of gram-negative

anaer-obes is observed, and particularly of Fusobacterium

nucleatum The latter organism has the unique ability

to co-aggregate with a wide variety of bacteria and

is believed to play a pivotal role in the maturation

of biofilm because it forms co-aggregation bridges

with both early and late colonizers As the biofilm

matures, a shift is observed toward a composition

of largely gram-negative morphotypes, including

rods, filamentous organisms, vibrios, and

spiro-chetes These shifts in the microbial composition of

biofilm are important because they correlate with the

development of gingivitis (inflammation of gingival

tissues)

Even though biofilms accumulate on restorative,

orthodontic, endodontic, and implant biomaterials,

the remainder of this section focuses on biofilms

that accumulate on the surfaces of restorative and

implant materials only The precise mechanisms of

bacterial adhesion and biofilm formation on the

sur-faces of dental materials have not yet been identified

in spite of decades of research effort but are accepted

to be complex processes that depend on a large

num-ber of factors In vitro studies have shown that the

adhesion of salivary proteins and bacteria at small

distances (5-100 nm) from the surfaces of

biomateri-als is influenced by a combination of Lifshitz-van der

Waals forces, electrostatic interactions, and acid-base

bonding Other properties such as substrate

hydro-phobicity, surface free energy, surface charge, and

surface roughness have commonly been investigated

in vitro for correlation with the number of adhering

bacteria Many of the above-mentioned surface

prop-erties are described in later chapters

The role of surface roughness in biofilm formation

has been widely investigated Smooth surfaces have

been shown to attract less biofilm in vivo than rough

surfaces It has also been observed that

hydropho-bic surfaces that are located supragingivally attract

less biofilm in vivo than more hydrophilic surfaces

over a 9-day period An increase in the mean surface

roughness parameter (Ra) above a threshold value

of 0.2 μm or an increase in surface free energy were

both found to result in more biofilm accumulation on

dental materials When both of those surface

proper-ties interact with each other, surface roughness was

observed to have a greater effect on biofilm

accumu-lation The creation of a rough restoration surface

caused by abrasion, erosion, air polishing or

ultra-sonic instrumentation, or a lack of polishing after the

fabrication of a restoration, has also been associated with biofilm formation

Bacterial adhesion in vivo is considerably reduced

by the formation of a pellicle, regardless of the position of the underlying substrate Pellicle forma-tion has also been shown to have a masking effect

com-on specific surface characteristics of biomaterials

to a certain extent Surfaces having a low surface energy were observed to retain the smallest amount

of adherent biofilm due to the lower binding forces between bacteria and substrata even after several days of exposure in the human oral cavity Recipro-cally, the higher surface energy of many restorative materials compared with that of the tooth surface could result in a greater tendency for the surface and margins of the restoration to accumulate debris, saliva, and bacteria This may in part account for the relatively high incidence of secondary (recurrent) carious lesions seen in enamel at the margins of resin composite and amalgam restorations

Investigations of oral biofilms on restorative materials can generally be divided into in vivo, in situ, and in vitro studies, with the latter comprising monospecies or multispecies investigations Biofilms that are formed on restorative materials can vary in thickness and viability In vivo and in situ studies of biofilm formation on dental materials have produced inconsistent results, and a trend for accumulation on materials has not been determined so far

Levels of cariogenic organisms (capable of

pro-ducing or promoting caries) such as Streptococcus mutans have been shown to be higher in biofilms adjacent to posterior resin restorations than in bio-films adjacent to amalgam or glass ionomer res-torations The formation of oral biofilms has been associated with an increase in the surface roughness

of resin composites, degradation of the material due

to acid production by cariogenic organisms, sis of the resin matrix, and a decrease in microhard-ness of the restoration’s surface Additionally, it has been theorized that planktonic bacteria can enter the adhesive interface between the restorative mate-rial and the tooth, leading to secondary caries and pulp pathology On the other hand, trace amounts

hydroly-of unpolymerized resin, resin monomers, and the products of resin biodegradation have been shown to modulate the growth of oral bacteria in the vicinity

of resin restorations All of these factors create a cycle

of bacteria-surface interaction that further increases surface roughness and encourages bacterial attach-ment to the surface, thereby placing the adjacent enamel at greater risk for secondary caries

Bacterial adhesion to casting alloys and dental amalgams has received limited attention in recent times Biofilms on gold-based casting alloys are reported to be of low viability, possibly due to the bacteriostatic effect of gold Biofilms on amalgam

are also reported to have low viability, which could

be attributed to the presence of the Hg(II) form of

mercury in dental amalgam Interestingly, amalgam

restorations have been shown to promote the

lev-els of mercury (Hg)-resistant bacteria in vitro and

in vivo Resistance to antibiotics, and specifically

tetracycline, was observed to be concurrent with

Hg-resistance in oral bacteria However, it is worth

noting that Hg-resistant bacteria were also found in

children without amalgam fillings or previous

expo-sure to amalgam

Information regarding the morphology of

bio-films on ceramic restorations is limited, although it

is generally accepted that ceramic crowns

accumu-late less biofilm than adjacent tooth structure The

recent demonstration of increased surface roughness

of zirconia surfaces in vitro after the use of hand and

ultrasonic scaling instruments could be theorized

to produce greater biofilm accumulation on

zirco-nia restorations subsequent to dental prophylaxis

procedures

Biofilms that adhere to denture base resins

pre-dominantly contain the Candida species of yeast

However, initial adhesion of bacteria such as

strep-tococci to the denture base may have to occur before

Candida species can form biofilms This is attributed

to the observation of bacteria on dentures within

hours and Candida species after days, and to the

ability of Candida species to bind to the cell wall

receptors in streptococci Biofilms on dentures have

commonly been associated with denture stomatitis

(chronic inflammation of the oral mucosa) in elderly

and immunocompromised patients Removal of

bio-films from dentures typically requires mechanical

and/or chemical means and is a significant clinical

problem because of biofilm adherence to the denture

base resins

The accumulation of biofilms on titanium and

titanium alloys that are used in dental implants has

received much attention because biofilms play a

sig-nificant role in determining the success of an implant

The sequence of microbial colonization and biofilm

formation on dental implants has been shown to be

similar to that on teeth, but differs in early

coloniza-tion patterns Several in vivo studies have confirmed

that a reduction in mean surface roughness (Ra)

of implant materials below the threshold value of

0.2 μm has no major effect on adhesion, colonization,

or microbial composition Compared to polished

titanium surfaces, titanium implant surfaces that

were modified with titanium nitride (TiN) showed

significantly less bacterial adhesion and biofilm

for-mation in vivo, thereby potentially minimizing

bio-film accumulation and subsequent peri-implantitis

Other contributing factors such as the

hydrophobic-ity, surface chemistry, and surface free energy of the

implant material have been found to play vital roles

in bacterial adhesion to dental implant materials In addition, the surface characteristics of the bacteria, the design of the implant and the abutment, and the micro-gap between the implant and abutment have also been shown to influence microbial colonization

on dental implants

The most common reason for the replacement of dental restorations is secondary caries at the gingi-val tooth-restoration margin It is estimated that 50%

to 80% of resin restorations are replaced annually in the United States alone The cost of replacing resto-rations is estimated to be in the billions of dollars worldwide, and the number and cost of replacing restorations is increasing annually Although bacte-riological studies of secondary caries indicate that its etiology is similar to that of primary caries, the mechanisms by which secondary caries occur are a focus of ongoing investigations

The removal of tenaciously adherent oral films from hard surfaces is crucial to caries control and is most effectively accomplished by mechanical brushing with toothpaste, especially in interproxi-mal regions and posterior teeth along with the use of adjunctive chemical agents Although tooth brushing has been associated with increased surface roughness

bio-of restorations over time due to the process bio-of wear, which could permit additional bacterial attachment

on the surface, mechanical removal has been shown

to be more effective than chemical intervention This

is because bacteria in biofilms are typically well tected from the host immune response, antibiotics, and antibacterials when embedded within a com-plex biofilm matrix Furthermore, most antimicrobial agents have commonly been tested against plank-tonic bacteria, which are killed by much lower con-centrations of antimicrobials than biofilm bacteria Chemical control of biofilms has also been limited

pro-by concerns regarding the development of resistant microorganisms resulting from the prolonged use of antimicrobials, and acceptance of the hypothesis that the microflora should not be eliminated but should instead be prevented from shifting from a favorable ecology to an ecology favoring oral disease

The accumulation of biofilms on glass ionomer and resin-modified glass ionomer biomaterials is a factor that has been associated with an increase in the surface roughness of those biomaterials Fluo-ride-releasing materials, and glass-ionomers and compomers in particular, can neutralize acids pro-duced by bacteria in biofilms Fluoride can provide cariostatic benefits and may affect bacterial metabo-lism under simulated cariogenic conditions in vitro Although the large volume of saliva normally pres-ent in the oral cavity is hypothesized to result in fluoride concentrations that are too low for cavity-wide antibacterial protection, the amount of fluoride released could theoretically be sufficient to minimize

PROBLEM 1

The enamel microstructure is unique among the

dental calcified tissues in that it is the hardest

tis-sue in the body Explain why its unique structure is

important to its function and how this has been used

in restorative treatments to provide reliable bonding

Solution

Enamel has the highest mineral content of the

calci-fied tissues providing high hardness and wear

resis-tance needed for mastication The small amount of

organic substances coats each crystallite and increases

toughness in comparison to pure apatite The very long

crystals are packed into keyhole–shaped enamel rods

that are separated by regions of higher organic

con-tent This structure etches differently when exposed

to acids, such as phosphoric acid, leading to a clean,

high-energy surface with varying roughness within

and between the rods

When infiltrated with resins and subsequently

polymerized, the resins form a tight and reliable bond

between the enamel structure and the resin

PROBLEM 2

Apatite is the mineral component of both enamel

and dentin, but there are important differences

between these critical forms of apatite What are the

critical differences and how do these differences affect

restorative dentistry?

Solution

Enamel and dentin mineral contain calcium-

deficient carbonate-rich hydroxyapatite However,

the crystallite size is different, with enamel having

much larger crystals that comprise 85 vol% of the

structure, compared to about 50 vol% in dentin The

larger surface area of the dentin apatite increases its

dissolution susceptibility when exposed to acids

This susceptibility is further increased because of the

higher carbonate content of the dentin mineral Thus

the dentin mineral is dissolved more rapidly than the

enamel mineral, and because there is less total eral in dentin than in enamel, the acid attack proceeds more quickly in dentin The dentin tubules also pro-vide pathways for this dissolution Therefore caries proceed more quickly once into dentin and gener-ally require surgical intervention and restoration In contrast, early enamel caries can be treated and the enamel remineralized

min-PROBLEM 3

Dentin tubules are an important structural feature

of dentin and are the pathways taken by odontoblasts during formation, starting from the DEJ and proceed-ing to the pulp chamber Why does this result in a dif-ferent number of tubules per unit area and difference

in moisture level with distance from the pulp?

Solution

Each odontoblast forms one tubule during tinogenesis Because the DEJ surface is larger than the surface of the pulp chamber, the tubules are more concentrated at deeper levels, resulting in an increased number of tubules per unit area Because tubules are filled with fluid and there is a positive pressure from the pulp, the higher number of (and somewhat larger diameter) tubules in deeper den-tin results in more moisture when deep dentin is cut than when superficial dentin is cut Thus deep dentin is inherently wetter than superficial dentin This has important implications for bonding because moisture may interfere with bonding procedures and there is less solid dentin in deeper dentin available for bonding

den-PROBLEM 4

Cavity preparations in the crown nearly always involve both enamel and dentin, and often at least some portions of the dentin may have been affected by caries What difficulties does this situation present for restorative dental treatments?

S E L E C T E D P R O B L E M S

demineralization in the tooth structure adjacent to

glass ionomer and resin-modified glass ionomer

res-torations In addition, glass ionomer materials can

be recharged by daily exposure to

fluoride-contain-ing dentifrices, thereby compensatfluoride-contain-ing for the

sig-nificant decrease in fluoride release that occurs over

time Interestingly, clinical studies have not clearly

demonstrated that fluoride-releasing restorative materials significantly reduce the incidence of sec-ondary caries as compared to non-fluoride-releasing biomaterials More studies are therefore needed to determine the impact of fluoride-releasing restora-tions on the development and progression of second-ary caries

These difficulties are particularly important in

bonding to such mixed substrates and are less

impor-tant for amalgam or crown preparations In bonding

applications we seek to make micromechanical bonds

to the enamel and dentin substrates This is generally

easier for enamel than dentin, and the enamel portions

of most preparations are in sound enamel

Further-more, enamel is not as sensitive to moisture content,

and the enamel can be thoroughly dried if necessary

to promote bonding Dentin is inherently wet with

moisture level increasing with depth in the crown

In dentin demineralized by etching or caries, drying

causes collapse of the exposed matrix, which makes

it difficult to penetrate by bonding agents In

addi-tion, cutting dentin results in smear layer formaaddi-tion,

which may interfere with bonding This may be of less

concern with some self-etching systems that

incorpo-rate smear layer or its remnants in the bonding layer

In addition, the dentin substrate may have been

altered by caries Such alterations may include loss of

mineral, making it more susceptible to etchants, or the

formation of a transparent layer that blocks the tubule

orifices and may restrict bonding Most research has

suggested that bond strengths to caries-altered dentin

are lower than to normal dentin

PROBLEM 5

The dentin-enamel junction (DEJ) is the starting

surface (starting line) for dentinogenesis After tooth

formation, the DEJ joins enamel and dentin and is

an important crack-arresting interface It is also an

important landmark for restorative procedures What

are the current concepts concerning this junction and

how it assists tooth function, minimizes tooth

frac-ture, and defines an important landmark to determine

treatment?

Solution

Enamel tends to be brittle and less tough than

dentin The dentin is needed to support and

distrib-ute stresses The DEJ joins these two different

calci-fied tissues and helps provide an integrated structure

that resists crack propagation from enamel to dentin

It appears to function this way by providing a

com-plex geometrical surface that helps deflect cracks and

provides the tooth structure with a more gradual

transition in properties from enamel to dentin Both

the geometrical complexity and the graduated

prop-erties enhance bonding between the tissues and

pre-vent abrupt transitions in mechanical properties Such

abrupt changes would lead to higher stresses at the

interface that otherwise would favor separation of the enamel from the dentin In addition, the DEJ is a key diagnostic marker in current practice because car-ies that progress past this junction are treated restor-atively, but lesions restricted to the enamel may be treated by remineralization In restorative treatments that require removal of tooth structure beyond the DEJ, the restored tooth is likely to be weaker and more prone to fracture

PROBLEM 6

List some of the factors that contribute to the increased accumulation of oral biofilms on resin com-posite restorations

Solution

Bacterial adhesion and biofilm formation on the surfaces of dental biomaterials are complex processes that depend on a large number of factors An increase

in the surface roughness of a restoration due to sion or erosion and factors affecting the degradation of the resin restoration, such as acid production by cario-genic organisms and hydrolysis of the resin matrix by saliva, are all capable of influencing biofilm accumula-tion on a resin restoration Insufficient polishing of a resin restoration has also been associated with biofilm formation Additionally, the release of trace amounts

abra-of unpolymerized resin and the products abra-of resin degradation can affect the growth of oral bacteria in the vicinity of resin composite restorations

bio-PROBLEM 7

You have a patient who has a large number of cervical restorations made of resin-modified glass ionomer The patient’s restorations and teeth were recently cleaned with an ultrasonic scaler What should you be concerned about?

Solution

The creation of a rough restoration surface by abrasion, erosion, air polishing, or ultrasonic instru-mentation has been associated with increased biofilm formation The restorations could be repolished or coated with a surface sealant (liquid polish) to rec-tify the roughening of the surfaces The restorations should be monitored periodically because clinical studies have not clearly demonstrated that fluoride-releasing dental biomaterials significantly reduce the incidence of secondary caries, and because secondary caries most commonly occur at the gingival margin of restorations

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Laboratory (In Vitro) Evidence

Creating the Plan Building the Restoration

As discussed in Chapter 2, restorative materials

are exposed to chemical, thermal, and mechanical

challenges in the oral environment The combination

of forces, displacements, bacteria, biofilm, fluids,

thermal fluctuations, and changing pH contribute to

the degradation of natural and synthetic

biomateri-als Each patient has a unique combination of these

factors When considering a new or replacement

restoration for a patient, the performance history of

the patient’s existing restorations can provide insight

into the prognosis of the new restoration The

perfor-mance of materials in controlled conditions, in vitro

and in vivo, is also useful when selecting materials

and predicting their service life Making the final

materials choices involves a complex

decision-mak-ing process that can be informed by principles of

product design

DESIGN CYCLE

Considering many factors and integrating many

specifications into one final product is a

require-ment of any object that requires fabrication In

prod-uct design, a cyclical approach of analyzing and

then testing problems is used to determine the best

design for the final production piece Three

catego-ries of problem-solving are used in the design cycle:

observe, plan, and build Then the steps are repeated

as the time, number of problems, and difficulty of

problems allow (Figure 3-1)

To illustrate how this process can be applied to

the design of a materials-sensitive product for dental

hygiene, we use the simple example of dental floss

The job this product needs to accomplish is removal

of interproximal plaque and debris All

interproxi-mal regions and surfaces are not the same Some

interproximal contacts are tight, others are open, and

some regions might have proximal restorations with varying degrees of marginal adaptation The devel-opment of a new dental floss product might start with the problem of a potential customer who has a two-surface posterior restoration with an overhang Current floss products on the market shred or tear

when flossing in such a region This main observation

is analyzed and deemed significant, because many people with this problem and similar problems could

be helped by a design change to this dental floss Multiple and varied ideas are generated to address the problem: (1) the dental floss cross section could

be a ribbon rather than a rope to ease the floss over the overhang; (2) the floss could be a single strand rather than a braid of multiple strands to reduce the number of surfaces on the floss that could catch; or (3) the floss could be made of a different material or

a slippery coating could be added to reduce friction (Note that all of these designs have been presented to consumers at one time or another.)

Based on these possible design changes, a plan is

made that incorporates a method or combination of methods that appear to be most promising in regard

to addressing the observed problem All of the sibilities could have merit, but by selecting those

pos-that address the observed problem most directly, one

can test the solutions most directly In this example,

we will say that the floss will be formed as a ribbon cross section and a change of material will be made

to reduce friction The new floss is built and tested in

simulated and actual environments

One cycle of our design process for a new tal floss has been completed We hope to find in our testing that we solved the observed problem That would be an effective solution What we may observe through testing our built product, however,

den-is that the material den-is too slippery to remove plaque effectively, or the ribbon is too wide to stay flat when drawn through the interproximal contact and into the gingival sulcus Based on these observations, a new plan is made, a new product version is created, and we find that we have completed another design cycle We repeat this process creating more refined versions of the product that provide more exacting solutions to the observed problems We also observe use of the product in as broad a range of consumer groups as possible to ensure the product addresses the needs of the target market

The design cycle for developing new products can

be used in the planning of restorations as well When selecting materials for a restoration, one observes the patient’s oral and medical condition and prioritizes the observed problems The observation data are integrated with valid materials performance data to create a plan of treatment A restoration is built and tested for occlusion, compatibility, esthetics, feel, and

so forth Adjustments are made in recurring observe,

Observe

FIGURE 3.1 The design cycle: Observe, Plan, Build, …

Repeat.

plan, build steps, refining the restoration to satisfy

both patient and clinician

EVIDENCE USED IN PRODUCT

DESIGN

The entire design cycle is based on evidence

Observation provides evidence about the history

of performance of existing materials and

solu-tions and identifies the job that new solusolu-tions

must perform The thoroughness of the

observa-tion phase depends on the skills and experience of

the designer In the plan phase, material properties

and characteristics and test data for performance

of materials in controlled conditions are added to

the observation data The build phase integrates

knowledge of the job or problem with the skill and

experience of the designer and considers

varia-tions in the operating condivaria-tions and properties

and known performance of the materials

With-out this systematic and integrative approach, the

design process would be haphazard and wasteful

The evidence-based design cycle just described is

analogous to evidence-based decision making in

health care and evidence-based dentistry

EVIDENCE-BASED DENTISTRY

The American Dental Association (ADA) defines

evidence-based dentistry as an approach to oral health

care that requires the judicious integration of

sys-tematic assessments of clinically relevant scientific

evidence relating to the patient’s oral and medical

condition and history, along with the dentist’s

clini-cal expertise and the patient’s treatment needs and

preferences (http://ebd.ada.org) This approach is

patient centered and tailored to the patient’s needs

and preferences Our goal is to practice at the

inter-section of the three circles (Figure 3-2)

Patient Evidence

Patient needs, conditions, and preferences are

considered throughout the diagnostic and treatment

planning process Observation of patient needs and

medical/dental history occurs first In this phase,

performance of prior and existing restorations, in

terms of success or failure, should be noted This

is often a good indicator of conditions in the oral

environment and the prognosis of success of similar

materials in this environment The patient’s facial

profile and orofacial musculature is a good indicator

of potential occlusal forces Wear patterns on occlusal

surfaces are indicators of bruxing, clenching,

occlu-sal forces, and mandibular movements Cervical

abfractions may indicate heavy occlusal contact accompanied by bruxing or occlusal interferences Erosion on anterior teeth suggests elevated levels of dietary acids, and generalized wear without occlu-sal trauma could involve a systemic disorder such

as gastroesophageal reflux disease (GERD) Any of these conditions would compromise the longevity of restorative therapy Unusually harsh environments require careful restoration design and selection of materials, sometimes different from the norm.The options for material to be selected then need

to be considered in accord with the problems and needs exhibited by the patient These data are found

in the scientific literature The integration of patient data and materials data helps make a more fully con-sidered plan for treatment

Laboratory (In Vitro) Evidence

When searching for scientific evidence, the best available evidence, usually compiled from a review

of the scientific literature, provides scientific dence to inform the clinician and patient The high-est level of validity is chosen to minimize bias These studies are typically meta-analyses of randomized controlled trials (RCTs), systematic reviews, or indi-vidual RCTs Lower levels of evidence are found in case studies, cohort studies, and case reports Labo-ratory studies are listed as “other evidence” because

evi-a clinicevi-al correlevi-ation cevi-an be mevi-ade only evi-as evi-an extrevi-ap-olation of the laboratory data The listing of bench

extrap-or labextrap-oratextrap-ory research as “other evidence” should not be construed as meaning that bench research is not valid The hierarchy of evidence as presented for evidence-based data (EBD) is based on human clinical data, for which bench data can only be a surrogate

When searching for scientific evidence, the best available, or most valid, data should be chosen New

Scientificevidence

Clinicianexperience andexpertise

Patient needs,conditions andpreferences

FIGURE 3.2 The elements of evidence-based dentistry.

material developments that are enhancements to

existing products are not required to undergo

clini-cal testing by the Food and Drug Administration

(FDA) Published laboratory or in vitro studies are

often the only forms of scientific evidence available

for materials This does not mean that no evidence

is available It is simply an indication that laboratory

studies should be admitted into evidence for making

the clinical decision (Table 3-1)

Researchers in dental materials science have

sought to correlate one or two physical or

mechani-cal properties of materials with clinimechani-cal performance

Although it is possible to use laboratory tests to rank

the performance of different formulations of the

same class of material, the perfect clinical predictor

remains elusive Often the comparison of

laboratory-based materials studies is difficult because of an

incomplete description of methods and materials

Researchers in dental materials are encouraged to

provide a complete set of experimental conditions in

their publications to enable the comparison of data

among studies This process will facilitate systematic

reviews of laboratory studies that can be used as a

source of scientific evidence when clinical studies are

not available

Every patient is unique, including the patient’s

oral environment and general physiology This

pro-vides a unique set of circumstances and challenges

for implementing successful materials choices in a treatment plan The elements of EBD and material properties should be considered as a system to pro-vide the best patient-centered care The observed evi-dence in an assessment of the patient, the analyzed evidence of the laboratory data, the experience of the clinician, and the needs and wants of the patient are all related and all impact the prognosis of the resto-ration Although it might be tempting to categorize

a patient’s needs by age, gender, or general clinical presentation, careful data gathering, planning, and analysis provides the best solution This assessment

is the basis for the complex process of oral tion (Figure 3-3)

rehabilita-CREATING THE PLAN

The plan phase integrates elements of

evidence-based decision making and a consideration of material properties and performance The process

of treatment planning is familiar to clinicians, but the practice of designing restorations with material properties in mind might not be done routinely To begin, performance requirements are analyzed The environment in which the restoration will serve is used as a modifier to the performance requirements For example, when treatment planning a three-unit

TABLE 3.1 Assessing the Quality of Evidence

Study Quality Diagnosis

High-quality diagnostic cohort study*

SR/meta-analysis or RCTs with consistent findingsHigh-quality individual RCT†

All-or-none study‡

SR/meta-analysis of good-quality cohort studiesProspective cohort study with good follow-up

Lower quality diagnostic cohort study or diagnostic case-control study

SR/meta-analysis of lower quality clinical trials or of studies with inconsistent findingsLower quality clinical trialCohort study

Case-control study

SR/meta-analysis of lower quality cohort studies or with inconsistent results

Retrospective cohort study

or prospective cohort study with poor follow-upCase-control studyCase seriesLevel 3: other

evidence disease-oriented evidence (intermediate or physiologic outcomes only), or case series for Consensus guidelines, extrapolations from bench research, usual practice, opinion,

studies of diagnosis, treatment, prevention, or screening

From Newman MG, Weyant R, Hujoel P: J Evid Based Dent Pract 7, 147-150, 2007.

*High-quality diagnostic cohort study: cohort design, adequate size, adequate spectrum of patients, blinding, and a consistent, well-defined reference standard.

† High-quality RCT: allocation concealed, blinding if possible, intention-to-treat analysis, adequate statistical power, adequate follow-up (greater than 80%).

‡In an all-or-none study, the treatment causes a dramatic change in outcomes, such as antibiotics for meningitis or surgery for appendicitis, which precludes study in a controlled trial.

SR, Systematic review; RCT, randomized controlled trial.